Section 44 AD - Tax Audit

If anyone is aware please do answer.

(a) If the business turnover is less than 01 crore and income is more than 6% (Digital Transactions), then can the assessee file ITR-3 form? Or is it mandatory to opt Section 44AD (Presumptive Scheme) and file ITR 4S?

(b) If the assessee doesnt opt Section 44AD and willing to show income less than 6%, then maintenance of Books and Tax Audit mandatory irrespective of Turnover (even for 15-20 Lakhs turnover)?